1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier or a printer, which forms images on recording materials using an electrophotographic process, and a relay control apparatus, and in particular to a power-feed path of a fixing apparatus that thermally fixes unfixed toner formed and carried on recoding materials.
2. Description of the Related Art
Conventionally, for electrophotographic image forming apparatuses, methods that heat and fix a toner image formed on a recording sheet (heat fixing methods) have been commonly adopted, and in particular, a method that brings a toner image into direct contact with a rotary member having a heat source therein and fixes the toner image has been in widespread use. As the heat source, a halogen heater, a ceramic heater, an IH heating, and so on are known, but all of them require so large amount of power as hundreds of watts.
Moreover, with an increase in demand for power saving, reducing standby electricity of image forming apparatuses has become an important issue. Thus, there has been proposed an image forming apparatus that raises fixing temperature at high speed by an on-demand fixing technique using a ceramic heater, and thus hardly requires standby electricity.
On the other hand, in such a fixing apparatus that raises fixing temperature at high speed, the temperature of a fixing heater abruptly rises, and it is thus important to quickly interrupt electric current to the fixing heater when an abnormal condition occurs. Moreover, to reliably interrupt electric current to the fixing heater, it is necessary to stop supplying electrical power to both ends of the fixing heater.
To stop the supply of electrical power to the fixing heater, a mechanical relay is commonly used. The relay uses a contact, and hence if the relay is repeatedly turned on and off, the contact may be welded due to age deterioration. If the contact of the relay is welded, electric current is passed through the fixing heater even when the relay is turned off, and thus power feeding to the fixing hearer does not stop, which may result in abnormal heating. To cope with this, there has been proposed a method that a zero cross detection circuit for detecting the presence or absence of input voltage is provided in a stage subsequent to the relay, and when a zero cross signal is output despite the mechanical relay being instructed to turn off, it is determined that contact fusion of the relay occurs (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2002-296955).
The above described method makes it possible to detect contact fusion of the relay by disposing the zero cross circuit in the stage subsequent to the relay.
However, when relays are disposed at respective both ends of the fixing heater, the zero cross circuit can detect contact fusion only when contacts of both relays are welded, and the zero cross circuit cannot detect contact fusion occurring in either one of the relays. For this reason, the above described method is insufficient in terms of safety.